I'm trying to make an object upgrade to more levels using only 1 button every time, so when that button is clicked, I just upgrade it to the object with an index +1 of the previous one, instead of checking which object needs to be upgraded..

C2 can't create instances from text string but you can do so with Nickname plugin: plugin-nickname-behavior-nickname_t74522 and store the name of your objects in text in the array. Then, refer to these text and create objects with respect to your plan.